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Radiation damage in SiO 2 /SiC interfaces

Radiation damage in SiO 2 /SiC interfaces. S. Dixit 1,2 , S. Dhar 2,3 , J. Rozen 1,2 , S. Wang 3 , S. T. Pantelides 3 , D. M. Fleetwood 4,3 , R. Schrimpf 4 and L. C. Feldman 1,2,3 1 Interdisciplinary Materials Science Program 2 Vanderbilt Institute of Nanoscale Science and Engineering

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Radiation damage in SiO 2 /SiC interfaces

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  1. Radiation damage in SiO2/SiC interfaces S. Dixit1,2, S. Dhar2,3, J. Rozen1,2, S. Wang3, S. T. Pantelides3, D. M. Fleetwood4,3, R. Schrimpf4 and L. C. Feldman1,2,3 1Interdisciplinary Materials Science Program 2Vanderbilt Institute of Nanoscale Science and Engineering 3Department of Physics & Astronomy 4Department of Electrical Engineering & Computer Science Vanderbilt University, Nashville, TN - 37235 MURI meeting June’06

  2. Outline Objective Motivation Introduction Experimental Results Conclusion MURI meeting June’06

  3. Objective Effect of ionizing radiation on SiO2/4H-SiC MOS devices. Comparison between as-oxidized and nitrided oxides. Comparison between positively-biased (electrons swept) and grounded irradiations. MURI meeting June’06

  4. Deep-space missions - Electronic switches and circuits High power and high temperature systems Concerns: Weight, efficiency and reliability Motivation Eg ~ 3.23 eV at RT Wide Band Gap ~ 4.5 Wcm-1s-1 ~ 2.0 MV cm-1 High thermal conductivity High breakdown field strength High power device 4H Silicon Carbide ~ 2.0 x 107 cm s-1 High current densities Inert Only compound semiconductor whose native oxide is SiO2 MURI meeting June’06

  5. Introduction SiO2/SiC interface is different from SiO2/Si Typical oxidation temperatures ~ 1100 - 1300 °C SiO2 Transition layer ~ 1nm N N N N N N N N N N N N N N SiC Thermal oxidation of SiC sub-oxide bonds oxycarbides free carbon Nitrided At Ec-E ~ 0.1 eV, Dit ~ 1013 cm-2 eV-1 ---> as-oxidized Dit ~ 1012 cm-2 eV-1 ---> nitrided MURI meeting June’06

  6. 10 keV X-rays 40 nm Sample preparation: Experimental n-type 4H-SiC SiO2 SiO2 thermally grown at 1150°C, 4 hrs NO Passivation at 1175°C, 2 hrs n-type 4H-SiC Sputter deposited Mo/Au dots Sputter deposited back contact Au back contact • Irradiation: • 10 keV X-rays, RT radiation • 31.5 krad(SiO2)/min dose rate • Function of dose • Function of positive bias [~1.5 MV/cm, ~2.3 MV/cm] MURI meeting June’06

  7. Grounded irradiation Biased irradiation As-oxidized samples Results • Positive charge buildup (like SiO2/Si) followed by a turnaround to net negative charge trapping • For high doses (>1Mrad(SiO2)), grounded samples show net negative charge while the biased samples show net positive charge (like SiO2/Si) MURI meeting June’06

  8. Biased irradiation Grounded irradiation Nitrided samples Results • Turn around also observed • Low dose regime (<1 Mrad) nitrided samples show more positive charge buildup • Indications of Fermi level Pinning was observed MURI meeting June’06

  9. At midgap, interface traps are charge neutral Vmg = Vot (Shifts in the CV curve) Vit = Vinv- Vmg (stretch-outs in the CV curve) EC Emg inv Si EF EV mg 4.7 eV 3.2 eV Bandgap argument EC Eg ~ 9.0 eV SiO2 EV Band diagram SiO2 and p-Si MURI meeting June’06

  10. 2.7 eV EC EC Emg Eg ~ 3.23 eV Emg Si EF EV 0.2 - 0.6 eV 4H-SiC EF EV 4.7 eV 3.2 eV 3.05 eV Time constant p(E) = (1/pTNv)exp(E-Ev)/kT Bandgap argument EC Eg ~ 9.0 eV SiO2 EV Band diagram SiO2 and p-type 4H-SiC MURI meeting June’06

  11. Vmg vs dose (Grounded samples) Results Nitrided/As-oxidized comparison • Positive charge trapping reported for the first time in grounded 4H-SiC capacitors for low dose irradiations • At higher doses, negative charge trapping (consistent with previous work 1) observed due to creation of oxide trapped charge and deep interface states (Ec-E~0.6 eV to mid-gap) 1 T. Chen, Z. Luo, J. D. Cressler, T. F. Isaacs-Smith, J. R. Williams, G. Chung, and S. D. Clark, “The effects of NO passivation on the radiation response of SiO2/4H-SiC MOS capacitors,” Solid State Electron., vol. 46, pp 2231-2235, 2002 MURI meeting June’06

  12. Vmg vs dose (Biased samples) Results Nitrided/As-oxidized comparison • Increased positive charge trapping observed in the case of nitrided samples under bias • This might suggest that the nitrided samples have less electron traps due to NO passivation as compared to the as-oxidized ones at low doses MURI meeting June’06

  13. Photo-CV results Photo-CV (Biased and grounded capacitors) • The hysteresis and prominent ledges are indicative of slow interface traps and near-interface (border) traps • Density of these traps are a factor of 2 higher for grounded as compared to positively-biased case, consistent with higher negative charge trapping MURI meeting June’06

  14. Conclusions • Significantly higher negative charge trapping for SiO2/SiC MOS devices compared to typical SiO2/Si • This can be attributed to • rapid buildup of interface statesanalogous to the hole-H interaction in SiO2/Si • wider bandgap of SiC which exposes the oxide related defects 3. Higher net positive charge buildup in the nitrided samples, consistent with a lower density of pre-existing interface and near-interface electron traps MURI meeting June’06

  15. Acknowledgements Work supported by the Air Force Office of Scientific Research through the MURI program and DARPA Thank you MURI meeting June’06

  16. Back up slides MURI meeting June’06

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